This invention relates to a device for pressing a sheet-shaped subject, and more particularly to a pressing device for pressing a recording sheet with rollers for use in an optical printer or a copying machine for fixing or developing an image formed thereon.
The prior pressing device of this kind usually comprises a pair of upper and lower pressing rollers, through which a recording sheet passes, and an image formed by toners temporarily adhered on the recording sheet is stably fixed thereon or a latent image formed on a photo and pressure sensitive recording sheet by exposing a corresponding image thereon is developed to a visible one with a pressure applied when passing through the rollers.
Conventionally, as illustrated in FIG. 3, the pair of rollers 1 and 2 are arranged so that the axes thereof cross each other at a certain angle .theta. (around 1.5 to 2 degrees). This is because, if the rollers are arranged to be perfectly in parallel with each other, the image on the recording sheet is non-uniformly fixed or developed due to the difference of the surface pressure arised in the direction of axes of the rollers due to the bending of the rollers caused when the recording sheet passes between the pair of rollers. The above non-uniform fixing or development of the image may be prevented by enlarging the diameters of the rollers to increase the rigidity thereof, but results in not only the increase of the weight of the rollers themselves but also the enlargement of the power source required to drive the rollers.
With the above constructed pressing devices, the above non-uniformness of the pressure applied on the recording sheet may be eliminated. However, as also illustrated in FIG. 3, the recording sheet P is tracted in the direction of the arrow E by the lower roller 1 while forced to be tracted in the direction of the arrow F by the upper roller 2 so that the recording sheet P receives tracting forces in the two different directions at the same time when passing through the pair of rollers 1 and 2. In this connection, it has often arised that the recording sheet P was discharged out of the rollers 1 and 2 in a wrinkled state and/or a distorted state.
Further, in case an image is formed on a transferring sheet which is a separate sheet by pressing the recording sheet and the transferring sheet in the state that both sheets are overlapped each other, it has often arised that the image transferred to the transferring sheet was blurred and/or the sheet became dirty due to the slipping between the two sheets.
As disclosed in the U.S. patent specification No. 439,920, there has been proposed to form a visible image by first optically forming a latent image on a recording sheet on which microcapsules enveloping chromogenic materials and photosensitive resins are coated and then transferring the image as a visible one to a transfer sheet on which color-developing agents are coated. More particularly, as illustrated in FIG. 4, the recording sheet 110 coated with a plurality of microcapsules, some s1 of which are softened by exposed to the light while others h1 of which are not-softened as being not exposed to light is pressed by a pair of rollers 110 and 120 together with a transferring sheet 120, on which a layer of color-developing agents 121 is formed. When pressed by the pair of rollers 110 and 120, the softened microcapsules s1 are ruptured and the chromogenic materials enveloped therein are streamed out of the microcapsules s1 and reacted with the color-developing agents 121 to form a color-developed area 121a on the surface of the transferring sheet 120. The not-softened microcapsules h1 are not ruptured so that the corresponding area on the surface of the transferring sheet 120 remains white if the original color of the transferring sheet 120 is white, thus the visible image is formed on the transferring sheet 120. Alternatively, the microcapsules may be hardened by exposed to the light and the unhardened microcapsules may be ruptured by pressed with a pair of rollers to form a visible image.
When pressing these sheets by the above constructed prior pressing device, since the direction of the tracting force applied by the lower roller 1 is different from that of the upper roller 2, the transferring sheet 120 is tracted in the different direction from that of the recording sheet 110, which results in slipping between both sheets 110 and 120 and therefore the image being blurred and/or dirty.